She has hundreds of folders :)) Local Folders appears right at the bottom of the list. This is one of the problems that started all this - after an update a short while ago the inbox disappeared.

This is a different story from the one you told us in the beginning. What you are telling now reminds me of several similar reports in the German and the French Thunderbird forums. https://forums.mozfr.org/viewtopic.php?t=157424

After the update to TB v143 some users with very long folder lists in their Local Folders could only see the upper part of this list. Might be the same issue as in https://support.mozilla.org/en-US/questions/1543105 and as in https://bugzilla.mozilla.org/show_bug.cgi?id=1990571 The solution was/is to downgrade to an earlier version of Thunderbird.

Before you downgrade start TB in the default profile, go to "Settings" > Updates and activate the radio button ◉ Check for updates, but let me choose whether to install them Otherwise Thunderbird would again update automatically to v144. Then download version 141 from this URLhttps://ftp.mozilla.org/pub/thunderbird/releases/141.0/ and install it over your actual TB version. Now you have to start Thunderbird 141 by launching it through the command line with the --allow-downgrade parameter: follow instructions on this page https://support.mozilla.org/en-US/kb/dedicated-profile-thunderbird-installation#w_what-happens-to-my-profile-if-i-downgrade-to-a-previous-version-of-thunderbird

Apologies for the break in communication, I was away from my computer for a while. I will look at this and do as you suggest. However the new version of TB seems to not allow downgrades as I have tried it.

I have also set her computer up as you say with the Check for Updates but TB still went ahead and automatically did the upgrade.

I will let you know how I get on.

However the new version of TB seems to not allow downgrades as I have tried it.

I just tested it on my Mac with the v144.0.1 I've been using for several weeks now. In Settings > General > Updates I verified that the radio button "Check for updates, but let me choose to install them" was still activated ◉ (see image) Then I deleted my TB 144.0.1 version from my "Applications" folder, downloaded v141.0 from https://ftp.mozilla.org/pub/thunderbird/releases/ installed it in my "Applications" folder and started it with the --allow-downgrade parameter from https://support.mozilla.org/en-US/kb/dedicated-profile-thunderbird-installation#w_what-happens-to-my-profile-if-i-downgrade-to-a-previous-version-of-thunderbird which I adapted to macOS.

Thunderbird v141.0 successfully started in its dedicated "Eclipse" profile, the same I had used before with the actual v144.0.1. When I go to Help > About Thunderbird the information window shows that I'm using TB 141.0 and proposes an update to v144.0.1 (second image). But as long as I don't hit the "Update to... " button there won't be any update and I can continue to work with v141.0 as long as I like.

I have also set her computer up as you say with the Check for Updates but TB still went ahead and automatically did the upgrade.

I don't understand that. You don't have to set up her PC not to update. It is in Thunderbird's Settings > General > Updates where you should disable updates.

Thanks, I will try this all tomorrow.

By "setting up her computer" I meant that I had set Thunderbird as you have stated. I did it twice and and both times TB automatically updated.

I will let you know what happens when I do as you have suggested.

Version 141.0 loaded fine with the inbox showing but with no folders again. I also tried version 142.0, which had worked before, and the same happened - inbox there but no folders.

In which of your two profiles are you running Thunderbird 141 or 142 actually? The hpp45uw3.default or the 98b52xm4.default-release?

To verify launch TB, go to Help > Troubleshooting Information > Profiles > about:profiles .... If the Profile: default-release is indicated as the "profile in use...." go to Profile: default and click on "Launch profile". Probably the second main window with the default profile will be hidden by the first main window, so you must "look behind". Do you see the missing folders in the default profile?

Ingwe ?